It is becoming increasingly clear that reversible acetylation of proteins is a signal directly controlling the activity of key cellular regulators. The enzymes controlling protein acetylation were identified as histone acetyltransferases (HATs) and histone deacetylases (HDACs). Following the discovery of HATs and HDACs, a number of non-histone proteins have been identified as substrates for these enzymes. HDACs play important roles in transcriptional regulation and pathogenesis of cancer through removing acetyl groups from histones and other transcriptional regulators. HDAC inhibitors case cell cycle arrest, differentiation and/or apoptosis of many tumors, suggesting their usefulness for chemotherapy and differentiation therapy. Since recent studies have revealed that HDACs are structurally and functionally diverse, it should be possible to develop inhibitors specific to individual HDACs as more promising agents for cancer therapy.